This invention relates to protective systems such as used to protect some components of gas turbine engines and, more particularly, to the protective-coating surface and the protective coating composition.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high-temperature durability of the components of the engine must correspondingly increase. Significant advances in high-temperature capabilities have been achieved through the formulation of nickel- and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, these components are often protected by an environmental and/or thermal-insulating coating, the latter of which is termed a thermal barrier coating (TBC) system. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as a thermal barrier coating (TBC), or topcoat, of TBC systems used on gas turbine engine components. The TBC employed in the highest-temperature regions of gas turbine engines is typically deposited by electron beam physical vapor deposition (EBPVD) techniques that yield a columnar grain structure which is able to expand and contract without causing damaging stresses that lead to spallation.
To be effective, TBC systems must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between ceramic topcoat materials and the superalloy substrates they protect. To promote adhesion and extend the service life of a TBC system, an oxidation-resistant bond coat is usually employed. Bond coats are typically in the form of overlay coatings such as MCrAlX (where M is iron, cobalt, and/or nickel, and X is yttrium or another rare earth element), or diffusion aluminide coatings. A notable example of a diffusion aluminide bond coat contains platinum aluminide (NiPtAl) intermetallic. When a bond coat is applied, a zone of interdiffusion forms between the substrate and the bond coat. This zone is typically referred to as a diffusion zone.
During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during engine service, bond coats of the type described above oxidize to form a tightly adherent alumina (aluminum oxide or Al2O3) layer or scale that protects the underlying structure from catastrophic oxidation and also adheres the TBC to the bond coat. The service life of a TBC system is typically limited by spallation at or near the interfaces of the alumina scale with the bond coat or with the TBC. The spallation is induced by thermal fatigue as the article substrate and the thermal barrier coating system are repeatedly heated and cooled during engine service.
There is a need for an understanding of the specific mechanisms that lead to the thermal fatigue failure of the protective system, and for structures that extend the life of the coating before the incidence of such failure. The present invention fulfills this need, and further provides related advantages.
The present invention provides an approach for fabricating an article protected by a protective system, and articles protected by the protective system. The life of the protective system is extended under conditions of thermal fatigue by delaying the onset of the alumina scale interface failure mode. The present approach is applicable to environmental-coating protective systems where there is no thermal barrier coating present. However, it realizes its greatest advantages when used in thermal barrier coating systems where the protective coating is a bond coat and a ceramic thermal barrier coating overlies the bond coat.
A method of fabricating an article having a protective coating thereon comprises the steps of providing an article substrate having a substrate surface, thereafter producing a flattened protective coating on the substrate surface by depositing a protective coating on the substrate surface, the protective coating having a protective-coating surface, and processing the protective coating to achieve the flattened protective-coating surface. The protective coating is thereafter optionally exposed to an environment wherein the protective-coating surface is controllably oxidized. The article substrate and protective coating have an average sulfur content of less than about 10 (more preferably less than 5, and most preferably less than 1) parts per million by weight at depths measured from the protective-coating surface to a depth of about 50 micrometers below the protective-coating surface. Optionally but preferably, a ceramic thermal barrier coating is deposited overlying the pre-oxidized protective-coating, so that the protective coating constitutes a bond coat for the thermal barrier coating.
The article substrate preferably is a nickel-base superalloy, and most preferably is a component of a gas turbine engine. The protective coating may be a diffusion aluminide protective coating such as a platinum aluminide protective coating, or it may be an overlay protective coating.
The protective coating may be flattened without removing material from the protective-coating surface, as by peening the protective coating. Alternatively, the protective coating may be flattened by removing material from the protective-coating surface, as by polishing the protective coating. Desirably, the step of processing the protective coating produces a protective-coating surface wherein an average grain boundary displacement height of the protective coating is less than about 3 micrometers, more preferably less than about 1 micrometer, even more preferably less than about 0.5 micrometer, and most preferably substantially zero, over at least about 40 percent of the surface area of the protective coating but more preferably over the entire surface area of the protective coating. Where the processing is accomplished by polishing, the average grain boundary displacement height may be substantially zero in the polished areas, where the polishing is to a mirror finish. In most cases, the step of processing the protective coating is performed after the step of depositing the protective coating is complete. In some cases, however, the steps of depositing the protective coating and processing the protective coating are performed concurrently. Additionally, it is preferred that at least about 40 percent, and more preferably all, of the surface of the protective coating is flattened to have a grain displacement height of less than about 3 micrometers, more preferably less than about 1 micrometer, even more preferably less than about 0.5 micrometer, and most preferably substantially zero.
The optional step of controllable oxidation preferably includes the step of heating the protective coating in an atmosphere having a partial pressure of oxygen of from about 10xe2x88x925 mbar to about 103 mbar, more preferably from about 10xe2x88x925 mbar to about 10xe2x88x922 mbar, at an oxidizing temperature of from about 1800xc2x0 F. to about 2100xc2x0 F., and for a time of from about xc2xd hour to about 3 hours. Most preferably, the controllable oxidation is performed by heating the protective coating to a pre-oxidation temperature of from about 2000xc2x0 F. to about 2100xc2x0 F. in a heating time of no more than about 45 minutes, preferably from about 1 to about 45 minutes, and more preferably from about 15 to about 35 minutes, and thereafter holding at the pre-oxidation temperature for a time of from about xc2xd hour to about 3 hours, in an atmosphere having a partial pressure of oxygen of about 10xe2x88x924 mbar.
An article having a protective coating thereon comprises an article substrate having a substrate surface, and a protective coating on the substrate surface. The protective coating has a protective-coating surface with an average grain boundary displacement height of less than about 5 micrometers (more preferably 1 micrometer, even more preferably 0.5 micrometer, and most preferably substantially zero) over at least about 40 percent (and preferably 100 percent) of the surface area of the article. The article substrate and the protective coating have an average sulfur content of less than about 10 parts per million by weight at depths measured from the protective-coating surface to a depth of about 50 micrometers below the protective-coating surface. These low sulfur levels may result from the manner in which the protective-coating is deposited. More commonly, however, sulfur is removed from the protective-coating surface by a desulfurization process after the protective coating is deposited. Preferably, a thermal barrier coating is deposited overlying the pre-oxidized protective coating, so that the protective coating constitutes a bond coat for the thermal barrier coating. Features discussed above in relation to the fabrication method may be used in conjunction with the article as well.
It has been known to employ a low-sulfur protective coating or bond coat, where the sulfur content is necessarily less than about 1 part per million by weight. Sulfur preferentially segregates to the interface between the protective coating and the alumina scale, accelerating the spalling of the alumina scale during thermal cycling. The reduction in the sulfur content of the protective coating can delay the onset of such a failure mechanism.
While this low-sulfur approach has proved useful in many instances, in other situations there was little if any improvement resulting from the low sulfur content of the protective coating. This lack of improvement resulted from the intervening failure mechanism of the development of mechanical convolutions in the alumina scale by ratcheting, which in turn resulted from the ridge-like structure of the protective-coating surface that leads to the initiation and propagation of mechanical damage in the ceramic just above the alumina scale and or within the alumina scale itself. In the present approach, the prominence of the ridge-like structure is reduced or eliminated by the flattening procedure. As a result, the onset of failure due to the development of convolutions is delayed, so that failure due to decohesion of the alumina scale from the bond coat becomes the life-limiting factor. Sulfur segregation, which is of great importance in scale adhesion, here plays a greater role in determining the ultimate failure mechanism of the protective structure. As a result, reducing the sulfur content becomes of greater importance, and the present invention provides for such a reduction.
Alternatively stated, failure of the protective coating may result from either of two mechanisms, the development of mechanical convolutions (and associated mechanical damage) or the degradation of the chemical adhesion of the scale to the bond coat, which is directly related to the chemical segregation of sulfur to the surface of the bond coat. The present approach addresses both mechanisms and takes steps to reduce their onset. The result is a longer-lived protective coating or, in the case of the thermal barrier coating system, the bond coat.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.